1. Technical Field
The present disclosure relates to a latent heat cold storage material that is used in storage and transportation of, for example, fresh foods. The latent heat cold storage material is filled into a cold storage vessel, and cooled in a freezer before being used.
2. Description of the Related Art
A mechanism of distributing products while maintaining a low-temperature state, a refrigerated state or a frozen state consistently from a production area to a consumption area is called a cold chain. The cold chain is now used in various fields including not only those of fresh products and frozen foods but also those of cut flowers, pharmaceuticals, electronic components and so on, but previous mechanisms have many problems. Particularly significant problems include energy costs for cooling apparatuses, an increase in CO2 emission, severity of work at cargo handling sites, and difficulty in managing different temperature zones. In particular, it has been pointed out that refrigeration and freezing trucks are provided with a cooling apparatus, and accordingly require a high transportation cost, and are mostly large trucks, thus being unsuitable for transportation of small lots. Meanwhile, in the case of transportation using dry ice, low-temperature products can be transported together with normal-temperature products to keep the cost low. However, there are inevitable problems that products are carried at −79° C., i.e. a sublimation temperature, and therefore frozen to be degraded, that products cannot be kept at a constant temperature, that dry ice must be discarded after being used only once, that CO2 is generated in vaporization, and so on.
As a technique for solving these problems, heat storage materials have been considered to be promising, and partially put into practical use. The heat storage material is a material which stores heat or cold heat in a substance, and effectively utilizes entry and exit of heat as required. Particularly, a heat storage material which mainly utilizes an exothermic/endothermic reaction associated with a phase change in a substance is called a latent heat storage material, and particularly, a heat storage material which stores cold heat beforehand, and is allowed to cool as required is called a latent heat cold storage material (hereinafter, also referred to simply as a “cold storage material”), but there is no clear distinction between the heat storage material and the cold storage material. Of latent heat cold storage materials, those having as a cold storage medium an inorganic substance-based material such as an inorganic salt or an inorganic hydrated salt have advantages that they have a higher heat conductivity, a larger latent heat amount and a smaller volume change in comparison with organic substance-based materials, and that they are incombustible. Among latent heat cold storage materials, an aqueous sodium chloride solution also has advantages that it has no toxicity and low reactivity, is readily available, and has moderate solubility and a eutectic temperature close to a frozen food preservation temperature. Therefore, these cold storage materials can be especially suitably used in refrigeration of foods, cooling during delivery, refrigeration of chemicals and pharmaceuticals, and cooling processes in food plants. PTL 1 describes a latent heat cold storage material of which melting temperature can be adjusted as desired using sodium chloride.
Cold storage materials are classified mainly into those for refrigeration and those for freezing depending on a use temperature range of the materials, and when a cold storage material for freezing is used, handling is easy because there is no possibility of releasing CO2 in vaporization unlike in dry ice, and there is no risk of low-temperature burns, and electric energy is required only during cooling of the cold storage material.
Conventional cold storage materials for freezing include aqueous solutions in which a plurality of electrolytes are dissolved, and specifically, for example, a mixed aqueous solution of sodium chloride and ammonium chloride as disclosed in PTL 2 is used.